1. Field of Invention
This invention relates to a mixed polymer electrolyte and to a battery which utilizes a mixed polymer electrolyte.
2. Description of Prior Art
The field of polymer electrolytes is experiencing vigorous activities in recent times due to significant theoretical interests as well as practical importance for the development of solid-state rechargeable Li batteries, sensors and electrochomic devices. A part of the current effort is concerned with development of polymer electrolyte-based high energy denisty solid-state Li batteries. The classical polymer electrolytes are composed of complexes of poly-(ethylene oxide), or PEO, with Li sales. The low conductivity, i.e.. &lt;10.sup.-7 .OMEGA..sup.-1, cm.sup.-1, of these polyolefin electrolytes at room temperature has limited their use to batteries operating at relatively high timepratures of about 100.degree. C. The low conductivity has also been a factor contributing to excessive dendritic growth, and consequent shorting, in Li/TiS.sub.2 abd Li/V.sub.6 O.sub.3 cells during cycling.
However, since the discovery that Li.sup.+ conductivity in the complexes of Li salts with inorganic polymers poly[bis(methoxyethoxyethoxide)phosphazene], or MEEP, is 2-3 orders of magnitude higher than that in the corresponding complexes of poly(ethylene oxide) (PEO), attempts have been made to utilize them in all-solid state Li batteries. A drawback of MEEP-(LiX).sub.n complexes is that at room temperature and above they are glutinous materials and slowly flow under pressure. This lack of dimensional stability of MEEP-based electrolytes has hindered their application in practical Li batteries and other solid-state devices. Several approaches have been pursued to enhance the mechanical properties of MEEP. These include chemical crosslinking of MEEP with poly(ethylene glycol), irradiation of MEEP or MEEP-(LiX).sub.0.25 complexes with various doses of .sup..alpha. Co .gamma.-ray, and the use of a porous, fiberglass support-matrix.
Other attempts to improve the dimensional stability of MEEP-(LiX).sub.n complexes have consisted of blending MEEP with high molecular weight PEO. Free-standing thin films with a slightly lower conductivity than that of MEEP-(LiX).sub.n could easily be cast from these blends.
Mixed polymer electrolytes composed of Li salt complexes of poly[bis-(methoxyethoxyethoxide)phosphazene], called MEEP, and poly(ethylene oxide), known as PEO, have shown excellent dimensional stability and high conductivity MEEP/PEO-(LiX).sub.n mixed polymer electrolytes, in which LiX =LiBF.sub.4 and LiClO.sub.4, have exhibited conductivities at 60.degree. C. which are comparable to the conductivity of PEO-(LiX).sub.n at 100.degree. C. Furthermore, the conductivity of these mixed electrolytes at room temperature is about 500-fold better than that of PEO-based electrolytes. These will translate into higher rate capability and/or lower temperature of operation for solid-state Li cells containing these mixed electrolytes Complexes containing LiAsF.sub.6, LiAlCl.sub.4, or LiCF.sub.3 SO.sub.3, exhibited lower conductivities than those containing LiBF.sub.4 or LiClO.sub.4. The higher conductivity of the mixed electrolytes compared to PEO complexes may be attributed to the higher degree of amorphous character introduced by the presence of MEEP.
A discussion of these mixed polymer electrolytes and the experimental test results which were obtained can be found in Journal of Electrochemical Society, Vol. 136, No. 12, December 1989 by K. M. Abraham, M. Alamgir and R. K. Reynolds.